1. Field
The disclosed embodiments relate generally to wireless communications, and more specifically to performing forward-link scheduling in a wireless communication system.
2. Background
Traditionally, wireless communication systems were required to support a variety of services. One such communication system is a code division multiple access (CDMA) system which conforms to the xe2x80x9cTIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d hereinafter referred to as IS-95. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention, and co-pending U.S. patent application Ser. No. 09/382,438, entitled xe2x80x9cMETHOD AND APPARATUS USING A MULTI-CARRIER FORWARD LINK IN A WIRELESS COMMUNICATION SYSTEM,xe2x80x9d each of which is incorporated by reference herein.
More recently, wireless systems such as the CDMA systems mentioned above have offered hybrid services, such as providing both wireless voice and data communications. To coordinate the implementation of such services, the International Telecommunications Union requested the submission of proposed standards for providing high-rate data and high-quality speech services over wireless communication channels. A preliminary proposal was issued by the Telecommunications Industry Association, entitled xe2x80x9cThe cdma2000 ITU-R RTT Candidate Submission,xe2x80x9d incorporated by reference herein and hereafter referred to as cdma2000. Various methods for transmitting non-voice data over fundamental and supplemental channels are disclosed in cdma2000.
In a CDMA system, a user communicates with the network through one or more base stations. For example, a user on a remote station (RS) may communicate with a land-based data source, such as the Internet, by transmitting data to a base station (BS) via a wireless link. A remote station may comprise a cellular telephone for mobile subscribers, a cordless telephone, a paging device, a wireless local loop device, a personal digital assistant (PDA), an Internet telephony device, a component of a satellite communication system, or any other component device of a communications system. The link between the RS and the BS is commonly referred to as the xe2x80x9creverse link.xe2x80x9d The BS receives the data and routes it through a base station controller (BSC) to the land-based data network. When data is transmitted from the BS to the RS, it is transmitted on the xe2x80x9cforward link.xe2x80x9d In CDMA IS-95 systems, the forward link (FL) and the reverse link (RL) are allocated to separate frequencies.
The remote station communicates with at least one base station during a communication. However, CDMA RSs are also capable of communicating with multiple BSs simultaneously, such as during soft handoff. Soft handoff is a process of establishing a new forward and reverse link with a new base station before breaking the old links with the previous base station. Soft handoff minimizes the probability of dropped calls, that is, where a call is inadvertently disconnected from the system. A method and apparatus for providing communications between an RS and more than one BS during the soft handoff process is disclosed in U.S. Pat. No. 5,267,261, entitled xe2x80x9cMOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. Softer handoff is a process of establishing a new forward and reverse link with a new sector of a current base station before breaking the old links with the previous sector.
Given the growing demand for wireless data applications, the need for very efficient voice and data wireless communication systems has become increasingly significant. One method for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, non-voice data or voice data is partitioned into code channel frames that are 20 msec wide with data rates as high as 14.4 kbps.
A significant difference between voice services and data services is the fact that voice services have stringent fixed delay requirements. Typically, the overall one-way delay of voice services must be less than 100 msec. In contrast, selectively planned data service delays, even above 100 msec, can be used to optimize the efficiency of the communication system. For example, error correction coding techniques that require relatively long delays can be used with data service transmissions.
Some parameters that measure the quality and effectiveness of data transmissions are the transmission delay required for transferring a data packet, and the average throughput rate of the system. As explained above, a transmission delay does not have the same impact in data or xe2x80x9cnon-voicexe2x80x9d communication as it does for a voice or xe2x80x9cvoice-dataxe2x80x9d communication. Still, delays cannot be ignored because they are an important metric for measuring the quality of the data communication system. The average throughput rate is reflective of the efficiency of the data transmission capability of the communication system.
Further, in a wireless communication system, capacity is maximized when the transmission energy for a signal is kept to a minimum value while satisfying the quality performance requirements for the signal. That is, the quality of transmitted voice-data or non-voice data cannot be significantly degraded when received. One measure of the quality of a received signal is the carrier-to-interference ratio (C/I) at the receiver. Thus, it is desirable to provide a transmission power control system that maintains a constant C/I at a receiver. Such a system is described in detail in U.S. Pat. No. 5,056,109 entitled xe2x80x9cMethod and Apparatus for Controlling Transmission Power in a CDMA Cellular Telephone System,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
It is well known that in cellular systems the C/I of any given user is a function of the location of the RS within a coverage area. In order to maintain a given level of service, TDMA and FDMA systems resort to frequency reuse techniques, i.e. not all frequency channels and/or time slots are used in each base station. In a CDMA system, the same frequency channel allocation is reused in every cell of the system, thereby improving the overall efficiency. The C/I associated with an RS determines the information rate that can be supported on the forward link from the base station to the user""s RS. An exemplary system for transmitting high rate digital data in a wireless communication system is disclosed in co-pending U.S. patent application Ser. No. 08/963,386, entitled xe2x80x9cMETHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,xe2x80x9d now U.S. Pat. No. 6,574,211, issued Jun. 3, 2003 to Padovani et al., assigned to the assignee of the present application and incorporated by reference herein.
Because the C/I associated with a RS determines the information rate that can be supported on the forward link, it is useful to know transmission information for each frequency channel used and historic C/I information. This information is commonly collected at the RS and messaged to the BS. But this messaging uses valuable system resources. What is needed is an invention that would eliminate such messaging requirements. Preferably, the BS transmission power levels on a first channel would be used to predict favorable slots for transmitting additional data on a second channel.
It is well known in the art that knowledge of a communication channel can be used to increase capacity in a CDMA system by transmitting mostly at times when channel conditions are good. See, e.g., S. W. Kim and A. Goldsmith, xe2x80x9cTruncated Power Control in Code Division Multiple Access Communications,xe2x80x9d Globecom (1997); R. Knopp and P. Humblet, xe2x80x9cMultiple-Accessing over Frequency-Selective Fading Channels,xe2x80x9d PIMRC (1995); A. Goldsmith and P. Varaiya, xe2x80x9cIncreasing Spectral Efficiency Through Power Control,xe2x80x9d ICC (1993). This technique is commonly referred to as xe2x80x9cwaterfilling.xe2x80x9d An issue that arises in cellular or PCS CDMA systems is fairness in that users nearer to a given BS may be favored in a waterfilling approach. Accordingly, there is a tradeoff between total throughput and fairness among users.
An algorithm based on priority given just by the carrier-to-interference ratio (C/I) would always give all the power to the user close to the BS with the best channel. This would maximize system throughput, but be unfair to users that are far from the BS. One solution, recently introduced by D. Tse and entitled xe2x80x9cForward-Link Multiuser Diversity Through Rate Adaptation and Schedulingxe2x80x9d (not yet published), attempts to compromise between throughput and fairness by including throughput monitoring that introduces fairness by raising the priority of users who do not transmit overly long. Nevertheless, a need exists in the art to provide an improved forward-link scheduling technique that compromises between fairness and system throughput and is suitable for multiple users.
Embodiments disclosed herein address the above-stated needs by providing methods extended to soft and softer handoff for scheduling transmit rates and transmit power levels for data on a supplemental channel used in a wireless communication system. Accordingly, in one aspect of the invention, a method of scheduling transmit rates and transmit power levels of data users in a wireless communication system during softer handoff includes transmitting signals between a base location having at least two sectors and a remote station via at least one first channel per sector, wherein the transmitted signals comprise voice-data, measuring at the base location a ratio of transmission power levels for the voice-data transmitted via the least one first channel per sector; determining a historical profile for the ratio of transmission power levels; and using the historical profile for the transmission power ratio levels to select a second channel transmission power level and data rate for transmitting additional data.
In another aspect, a method of scheduling transmit rates and transmit power levels of data users in a wireless communication system includes, transmitting signals between a base location and a remote station via at least one first channel, wherein the transmitted signals comprise voice-data, measuring at the base location transmission power levels for the voice-data transmitted via the least one first channel, determining a historical profile for the transmission power levels, using the historical profile for the transmission power levels to select a second channel transmission power level and data rate for transmitting additional data, selecting a soft handoff power level and transmission rate based on average required power, and transmitting continuously to the user at the soft handoff power level and transmission rate during soft handoff.